The mammalian renal system serves primary roles both in the removal of catabolic waste products from the bloodstream and in the maintenance of fluid and electrolyte balances in the body. Renal failure is, therefore, a life-threatening condition in which the build-up of catabolites and other toxins, and/or the development of significant imbalances in electrolytes or fluids, may lead to the failure of other major organs systems and death. As a general matter, renal failure is classified as “acute” or “chronic”. As detailed below, acute and chronic renal failure are debilitating and life-threatening diseases for which no adequate treatments exist to delay, and/or reverse kidney structural alterations associated with the disease.
Acute renal failure (ARF) is usually caused by an ischemic or toxic insult that results in an abrupt decline in renal functions. The kidneys are highly susceptible to ischemia and toxicants because of their unique anatomic and physiologic features. The large renal blood flow (approximately 25% of the cardiac output) results in increased delivery of blood-borne toxicants to the kidney as compared to other organs. The renal cortex is especially susceptible to toxicant exposure because it receives 90% of renal blood flow and has a large endothelial surface area due to the numerous glomerular capillaries. Within the renal cortex, the proximal tubule (the S3 segment or “pars recta”) and the epithelial cells of the thick ascending arm of the loop of Henle, are most frequently affected by ischemic and toxicant-induced injury because of their solute transport functions and high metabolic rates. As water and electrolytes are reabsorbed from the glomerular filtrate, tubular epithelial cells can be exposed to increasingly high concentrations of toxicants. Similarly, in the medulla the counter-current multiplier system may concentrate toxicants. Toxicants that are either secreted or reabsorbed by tubular epithelial cells (such as gentamicin) may accumulate in high concentrations within these cells. Finally, the kidneys also play a role in the biotransformation of many drugs and toxicants. Biotransformation usually results in the formation of metabolites that are less toxic than the parent compound; however, in some cases (such as oxidation of ethylene glycol to glycolate and oxalate) the metabolites are more toxic.
ARF has three distinct phases, which are categorized as initiation, maintenance, and recovery. During the initiation phase, therapeutic measures that reduce the renal insult (e.g., fluid therapy) can prevent the development of established ARF. The maintenance phase is characterized by tubular lesions and established nephron dysfunction. The recovery phase of ARF occurs when renal function improves subsequent to nephron repair and compensatory hypertrophy. Tubular lesions may be repaired if the tubular basement membrane is intact and viable cells are present. In addition, functional and morphologic hypertrophy of surviving nephrons can, in some cases, adequately compensate for decreased nephron numbers. Even if renal functional recovery is incomplete, adequate function may be re-established in some cases. More commonly, however, tubular damage is severe and irreversible and a large percentage of animals die or are euthanized in the maintenance phase of ARF.
Despite tremendous efforts to decipher the cellular and molecular pathogenesis of ARF during the past decades, no effective treatment is currently available and the incidence of mortality remains very high in veterinary medicine. At least two retrospective studies have documented the poor prognosis associated with ARF in dogs. In a study of hospital acquired ARF, the survival rate was 38%, whereas in another study of all types of ARF, the survival rate was 24%. Thus, there is an un-met medical need for improved prevention and/or treatment of ARF.
Chronic renal failure (CRF) may be defined as progressive, permanent and significant reduction of glomerular filtration rate (GFR) due to significant and continuing loss of nephrons. CRF typically begins from a point at which a chronic renal insufficiency (i.e., a permanent decrease in renal function of at least 50-60%) has resulted from some insult to the renal tissues, which has caused a significant loss of nephron functional units. The initial insult may not have been associated with an episode of acute renal failure. Irrespective of the nature of the initial insult, CRF manifests a “final common path” of signs and symptoms as nephrons are progressively lost and GFR progressively declines. This progressive deterioration in renal function is slow and seemingly inevitable, typically spanning several months to years in canine and feline subjects and many decades in human patients.
The early stage of CRF typically begins when GFR has been reduced to approximately one-third of the normal level (e.g., 30-40 ml/min for an average human adult). As a result of the significant nephron loss, and in an apparent “attempt” to maintain the overall GFR with fewer nephrons, the average single nephron GFR (SNGFR) is increased by adaptation of the remaining nephrons at both the structural and functional levels. One structural manifestation of this adaptation that is readily detectable by microscopic examination of biopsy samples is a “compensatory hypertrophy” of both the glomeruli and the tubules of the kidney, a process that actually increases the volume of filtrate which can be produced by each remaining nephron by literal enlargement of the glomeruli and tubules.
As a result of the hypertrophy or dilatation of the collecting ducts, the urine of subjects with CRF often contains casts which are 2-6 times the normal diameter (referred to herein as “broad casts” or “renal failure casts”. The presence of such broad casts aids in diagnosis of CRF. At the same time, there are functional changes in the remaining nephrons, such as decreased absorption or increased secretion of normally excreted solute, which may be responses to hormonal or paracrine changes elsewhere in the body (e.g., increasing levels of parathyroid hormone (PTH) in response to changes in serum levels of calcium and phosphate).
These adaptations in the early stage CRF are not successful in completely restoring GFR or other parameters of renal function and, in fact, subject the remaining nephrons to increased risk of loss. For example, the increased SNGFR is associated with mechanical stress on the glomerulus due to hypertension and hyperperfusion. The loss of integrity of podocyte junctures leads to increased permeability of the glomerulus to macromolecules or “leakiness” of the glomerular capsule. Proliferative effects are also observed in mesangial, epithelial and endothelial cells, as well as increases in the deposition of collagen and other matrix proteins. Sclerosis of both the glomeruli and tubules is another common symptom of the hypertrophied nephrons and the risk of coagulation in the glomerulus is increased. In particular, these adaptations of the remaining nephrons, by pushing the SNGFR well beyond its normal level, actually decrease the capacity of the remaining nephrons to respond to acute changes in water, solute, or acid loads, and therefore actually increase the probability of additional nephron loss.
As CRF progresses, and GFR continues to decline to less than 10% of normal (i.e., around 5-10 ml/min in humans), the subject enters into end-stage renal disease (ESRD). During this phase, the inability of the remaining nephrons to adequately remove waste products and maintain fluid and electrolyte balance, leads to a rapid decline in which many organ systems, and particularly the cardiovascular system, may begin to fail. At this point, renal failure will rapidly progress to death unless the patient receives renal replacement therapy (i.e., chronic hemodialysis, continuous peritoneal dialysis, or kidney transplantation).
The management of CRF must be conducted to ameliorate all identifiable clinical, metabolic, endocrine and biochemical consequences induced by renal failure including, but not limited to, azotemia, nutritional inadequacies, hypoproliferative anaemia, disordered mineral metabolism, electrolyte disturbances, metabolic acidosis, proteinuria, disordered water metabolism, systemic hypertension and the progression of renal injury through interstitial fibrosis that is considered to be the commonly converging outcome of CRF regardless of the specific etiology.
While tremendous progress has been made during the last decade to address several clinical, metabolic, endocrine and biochemical consequences of CRF, the therapy of clinically chronic fibrosis remains extremely challenging and therefore the long-term medical control of renal disease remains an important un-met therapeutic need. Currently, most advanced therapy targeting the reduction of renal disease-associated fibrosis is focused on the reduction of the activity of the renin-angiotensin system (RAS). Although this strategy has been shown to slow the disease evolution, its efficacy remains partial and it does not completely halt the progression of chronic fibrosis in experimental and clinical conditions. This is probably because many factors other than RAS contribute to the pathogenesis of CRF associated fibrosis.
The prevalence of CRF in cats and dogs is increasing. For every 1000 cats evaluated in 1980 in the US, four had renal failure regardless of age. By 1990, the number of reported cases of renal failure has quadrupled with 16 cases identified for every 1000 cats examined. For cats older than 15 years of age, 153 cases of renal failure were diagnosed in 1990 for every 1000 examinations. The increase in prevalence of renal failure in aging cats may reflect an increase in veterinary care sought by owners as well as greater efforts by veterinarians to detect the disease. Whatever the reason, these findings emphasize the emerging awareness and importance of CRF in older animals. The most frequent etiologies of CRF in companion animals include, but are not limited to, idiopathic chronic interstitial nephritis, irreversible ARF, familial renal dysplasia or aplasia, congenital polycystic kidney disease, amyloidosis, glomerulonephritis, hypercalcemia, bilateral hydronephrosis, leptospirosis, pyelonephritis, nephrolithiasis bilateral, Falconi-like syndrome, hypertension, renal lymphosarcoma.
In human medicine, approximately 600 patients per million receive chronic dialysis each year in the USA, at an average cost approaching $60,000-$80,000 per patient per year. Of the new cases of end-stage renal disease each year, approximately 28-33% are due to diabetic nephropathy (or diabetic glomerulopathy or diabetic renal hypertrophy), 24-29% are due to hypertensive nephrosclerosis (or hypertensive glomerulosclerosis), and 15-22% are due to glomerulonephritis. The 5-year survival rate for all chronic human dialysis patients is approximately 40%, but for patients over 65, the rate drops to approximately 20%. Therefore, a need remains for treatments to prevent the progressive loss of renal function which has caused almost 200,000 human patients in the USA alone to become dependent upon chronic dialysis, and which results in the premature deaths of tens of thousands each year.
In light of the fact that specific morphogens and/or growth factors that exhibit renotropic properties and promote tubular repair and recovery of renal function have been recently identified, it is conceivable that some of these molecules could have the potential to be used as therapeutic agents for the prevention and/or treatment of ARF and/or CRF. One such agent is Bone Morphogenetic Protein-7 (BMP-7, or Osteogenic Protein-1, OP-1), which is a member of the Transforming Growth Factor-β (TGF-β) superfamily. BMP-7 binds to activin receptors types I and II, but not to TGF-β receptors type I, II and III. Monomeric BMP-7 has a molecular weight of 17 to 19 kDa and was originally identified by its ability to induce ectopic bone formation. BMP-7 polypeptide is secreted as a homodimer with an apparent molecular weight of approximately 35-36 kDa. Recently, BMP-7 has been shown to be a key morphogen during nephrogenesis. Renal expression of BMP-7 continues in mature kidneys, especially in medullary collecting ducts. Renal tubules also express BMP-7 receptors. In animal models of ARF and CRF, renal expression of BMP-7 is significantly down-regulated and the administration of recombinant BMP-7 protein has been reported to accelerate renal recovery, an effect that was associated with less interstitial inflammation and programmed cell death.
However, because BMP-7 has a short half live in vivo (approximately 30 min), maintenance of a sustained level of exogenous protein in the circulation following injection of the purified protein requires multiple short-interval administrations, creating a very significant practical challenge. The cost of such a multi-injection therapy is too high to be applicable in veterinary medicine. Although gene delivery has been successfully promoted as an alternative to protein therapy for various diseases treatment, it's applicability for ARF and/or CRF prevention and/or treatment through BMP-7 polypeptide expression in vivo has not been proposed previously, and its potential effectiveness remains uncertain. Indeed, the low molecular weight of the BMP-7 homodimer (i.e., approximately 35 kDa) would theoretically allow for rapid glomerular filtration. Whether or not levels of BMP-7 expressed in vivo could reach therapeutically effective plasma concentrations cannot be predicted or determined from the existing literature. To further complicate the evaluation of in vivo-expressed BMP proteins, results can be variable depending on the immune status of the treated animal, with significant differences between immune competent and incompetent animals. Thus, when considered collectively as a whole, the literature does not teach whether levels of BMP-7 expressed in vivo could reach plasma concentrations that would be therapeutically useful.
Citation or identification of any document in this application does not constitute an admission that such document is available as prior art to the present invention.